Triggering Mechanisms for Motor Actions: The Effects of Expectation on Reaction Times to Intense Acoustic Stimuli

Abstract

Motor actions can be released much sooner than normal when the go-signal is of very high intensity (> 100dBa). Although statistical evidence from individual studies has been mixed, it has been assumed that sternocleidomastoid (SCM) muscle activity could be used to distinguish between two neural circuits involved in movement triggering. We summarized meta-analytically the available evidence for this hypothesis, comparing the difference in premotor reaction time (RT) of actions where SCM activity was elicited (SCM+ trials) by loud acoustic stimuli against trials in which it was absent (SCM-trials). We found ten studies, all reporting comparisons between SCM+ and SCM-trials. Our mini meta-analysis showed that premotor RTs are faster in SCM+ than in SCM-trials, but the effect can be confounded by the variability of the foreperiods employed. We present experimental data showing that foreperiod predictability can induce differences in RT that would be of similar size to those attributed to the activation of different neurophysiological pathways to trigger prepared actions. We discuss plausible physiological mechanisms that would explain differences in premotor RTs between SCM+ and SCM-trials.

Neural gain induced by startling acoustic stimuli is additive to preparatory activation

“…At the 65th percentile, mean RT = 162.64 ms, SD = 24.63. A recent meta‐analysis comparing RTs based on SCM activity reports a mean difference in RT latency between SCM+ and SCM − trials of −16.9 ms (Leow et al, ). Our results indicate a mean difference of −25.57 ms between fast and slow latency responses.…”

“…It has been argued that there are two potential mechanisms that underlie responses to a startling stimulus. We conducted a CDF analysis (see Leow et al, ) in an effort to capture these two potential mechanisms and determine whether longer latency responses masked our ability to observe differences between muscles in their shortening of RT. Our analysis indicated no difference in response latency between muscle types in the fastest percentile of RT analyzed (35%).…”

“…To test this, a cumulative distribution function (CDF) was used to examine the effects of muscle type, as well as required force, at the 35th and 65th percentiles of RT for probe trials of both intensities. As reported by Leow et al (), 35th and 65th percentiles were chosen for the CDF based on mean RT latencies of trials in the presence (35th percentile) or absence (65th percentile) of SCM activity reported by Honeycutt et al (). While the presence or absence of SCM activity has previously been used in the literature in an attempt to separate responses that may or may not activate neurophysiological mechanisms responsible for the StartReact effect, this is not always reliable (Leow et al, ; Marinovic & Tresilian, ).…”

“…As reported by Leow et al (), 35th and 65th percentiles were chosen for the CDF based on mean RT latencies of trials in the presence (35th percentile) or absence (65th percentile) of SCM activity reported by Honeycutt et al (). While the presence or absence of SCM activity has previously been used in the literature in an attempt to separate responses that may or may not activate neurophysiological mechanisms responsible for the StartReact effect, this is not always reliable (Leow et al, ; Marinovic & Tresilian, ). For example, short latency responses indicative of the StartReact effect can be observed in the absence of SCM activity (Castellote, Kofler, Mayr, & Saltuari, ; Valls‐Solé, Kofler, Kumru, Castellote, & Sanegre, ).…”

“…Furthermore, longer latency responses can occur in the presence of SCM activity (Marinovic & Tresilian, ). Therefore, examining trials based on RT percentile latencies, rather than the presence or absence of surface SCM activity, may be a more reliable method of capturing these two potential mechanisms in response to intense acoustic stimuli (Dean & Baker, ; Leow et al, ; Marinovic & Tresilian, ).…”